As is commonly known, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored in the form of a data pattern. Optical discs may be read-only type, where information is recorded during manufacturing, which information can only be read by a user. The optical storage disc may also be a writable type, where information may be stored by a user.
For writing information in the storage space of the optical storage disc, or for reading information from the disc, an optical disc drive comprises, on the one hand, rotating means for receiving and rotating an optical disc, and on the other hand optical means for generating an optical beam, typically a laser beam, and for scanning the storage track with said laser beam. Since the technology of optical discs in general, the way in which information can be stored in an optical disc, and the way in which optical data can be read from an optical disc, is commonly known, it is not necessary here to describe this technology in more detail.
For rotating the optical disc, an optical disc drive typically comprises a motor, which drives a hub engaging a central portion of the optical disc. Usually, the motor is implemented as a spindle motor, and the motor-driven hub may be arranged directly on the spindle axle of the motor.
For optically scanning the rotating disc, an optical disc drive comprises a light beam generator device (typically a laser diode), an objective lens for focussing the light beam in a focal spot on the disc, and an optical detector for receiving the reflected light reflected from the disc and for generating an electrical detector output signal.
During operation, the light beam should remain focused on the disc. To this end, the objective lens is arranged axially displaceable, and the optical disc drive comprises focal actuator means for controlling the axial position of the objective lens. Further, the focal spot should remain aligned with a track or should be capable of being positioned with respect to a new track. To this end, at least the objective lens is mounted radially displaceable, and the optical disc drive comprises radial actuator means for controlling the radial position of the objective lens.
In many disc drives, the orientation of the objective lens is fixed, i.e. its axis is directed parallel to the rotation axis of the disc. In some disc drives, the objective lens is pivotably mounted, such that its axis can make an angle with the rotation axis of the disc.
For any reason, the optical disc may suffer from tilt. Tilt of the optical disc can be defined as a situation where the storage layer of the optical disc, at the location of the focal spot, is not exactly perpendicular to the optical axis. The tilt can have a radial component and a tangential component. As illustrated in FIG. 6, the radial component (radial tilt) is the angular component β of the deviation in a plane oriented transversely to the track to be read (i.e. along the radial direction R) and transversely to the data carrier, while the tangential component (tangential tilt) is defined as the angular component α of the deviation in a plane oriented parallel to the track (i.e. along the tangential direction T) to be read and transversely to the data carrier. Tilt can be caused by the optical disc being tilted as a whole, but is usually caused by the optical disc being warped, and as a consequence the amount of tilt depends on the location on disc. Especially systems, which have a relatively large numerical aperture (NA), are sensitive to disc tilt. Therefore, tilt compensation mechanisms have been developed. Typically, in a disc drive apparatus having tilt compensation, at least the objective lens is mounted pivotably, and the optical disc drive comprises tilt actuator means for controlling the tilt position of the objective lens so that the laser beam remain locally perpendicular to the disc surface. Alternatively, it is possible that the orientation of the disc itself is corrected.
For attenuating the effect of the disc tilt, there is thus a need for defining a method of measuring the optical disc tilt.
It is possible to measure the tilt with a separate tilt sensor. However, such solution would involve additional hardware and increased costs.
It has already been proposed in prior art to process an electrical output signal from the optical detector in order to obtain a tilt measuring signal indicating the tilt angle. Based on such a tilt measuring signal, a tilt controller can control the tilt actuator means in such a way that the tilt angle is reduced or even made zero.
The Japanese patent JP-2000 076 679-A discloses a combi-drive intended to read and write data on optical discs having different formats in using a plurality of different light beams. A first light beam, referred to as data beam, is used for the writing/reading operation. A second light beam, referred to as tilt beam, is used for tilt measurement. This combi-drive comprises means for measuring the tilt. For this purpose, the tilt beam is modulated with a predetermined modulation frequency, resulting in an electrical tilt-indicative signal component having the same frequency. A band-pass filter is thus used to derive this signal component, which is further processed for measuring tilt. This technique has some disadvantages.
On the one hand, this technique requires the use of at least a beam modulator, a band-pass filter, and a peak detector, which adds to the complexity and cost of the apparatus.
On the other hand, the tilt beam causes an electrical signal having modulation as well as a continue component, which may affect error detection based on the read signal output. The amount of influence is not constant but depends on tilt direction and magnitude, therefore the required compensation of this effect is difficult.
Further, the continuous use of the tilt beam adds to the power dissipation of the apparatus.